Current mirrors are often used in analog circuits to generate an output current proportional to an input current. The proportionality constant of the output and input current in current mirrors is commonly known as mirroring ratio. Generally, a simplest current mirror circuit can be realized only through employing two MOS (Metal Oxide Semiconductor) transistors. As CMOS (Complementary MOS) technology progresses into ultra deep submicron (UDSM) era, the MOS transistor dimensions are getting aggressively scaled down. In the present day, 90/65 nm, CMOS technology, due to the very thin gate oxide thickness, the MOS input impedance is no longer only capacitive. The gate impedance now has a dominant resistive component, which can be attributed to gate leakage. Generally, in a Bipolar Junction Transistor, the base current determines the collector current, but in the case of a MOS transistor the drain current is not determined by the gate current as MOS transistors could be broadly modeled as a voltage-controlled current source. In such an instance, one of the basic analog circuits that gets affected during operation is the current mirror. In MOS circuits, a simple diode connected transistor normally behaves as a very good current mirror. In such a simple current mirror circuit gate-leakage can lead to an error in the output current that is proportional to the mirroring ratio. It is often desired that the current mirror has a good accuracy with a small voltage across the current mirror, commonly referred as headroom. Generally, the short channel MOS transistor has lower output impedance resulting in mirroring error due to difference in the drain voltages of the diode and mirror transistors. The mirroring error due to the lower output impedance of the transistor is known as systematic error.
To overcome this problem, a conventional cascoding method employs four MOS transistors to complete the current mirror circuit. The cascoded current sources in this conventional method require more headroom when compared to the simple two transistor mirroring technique. These architectures minimize the systematic error. But most of the conventional cascoded current mirrors or simple MOS diode current mirrors suffer from large errors in the output current if there is significant gate leakage. Other conventional techniques use either an amplifier or a source follower between the mirroring transistors, which can also be used to address gate leakage problem. However, using the source follower, even though results in being gate leakage insensitive it still requires significant input headroom. There are other conventional techniques that use feedback amplifiers which require more area and power.